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New superconducting state verified in experiments

Superconductors are materials that conduct electrical currents without any loss below a certain temperature. Normally, high magnetic fields destroy the superconductivity, turning the material into a normal conductor. Novel experiments on organic superconductors revealed a new superconducting phase between the normal conducting and the superconducting state, which was predicted in theory already in 1964. Scientists of the Universities of Geneva/Switzerland, Braunschweig/Germany, Osaka/Japan, and of the Grenoble High Magnetic Field Laboratory in France as well as of the Dresden High Magnetic Field Laboratory of the Forschungszentrum Dresden-Rossendorf were involved in these recent investigations.

Superconductors have no electrical resistance at low temperatures. They are for example applied for magnet coils in magnetic resonance scanners or in particle accelerators. Each superconducting material becomes a normal conductor beyond a certain magnetic field. However, for some materials a new superconducting hybrid phase between the normal and the superconducting state occurs in high magnetic fields and at low temperatures. In this phase, parts of the material stay superconducting whereas other parts turn into the normal-conducting state. Due to this hybrid state the superconductivity can survive also in very high magnetic fields. This state preferably appears in stacked materials that consist of ultra-thin conducting and insulating layers.

Prof. Peter Fulde from the Max Planck Institute for the Physics of Complex Systems in Dresden and Prof. Richard Ferrell predicted the existence of this special superconducting state in 1964. Already at that time it was characterized by a spatial modulation of the superconductivity. At about the same time, two further researchers independently predicted the same phase. Therefore, the state is called Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state.

First successful experiments on an organic superconductor were performed in static magnetic fields at the Grenoble High Magnetic Field Laboratory in 2007. This material was expected to become a normal conductor at about 22 Tesla (Tesla is the unit for the magnetic flux density and a measure for the strength of the magnetic field). But if the magnetic field is applied parallel to the organic molecule layers the superconductivity can survive also in much higher fields due to the formation of the hybrid phase. The results have been published in ‘Physical Review Letters’.

Recently, a second series of experiments has been finished. Again, researchers of the Dresden High Magnetic Field Laboratory at the Forschungszentrum Dresden Rossendorf contributed to these investigations. The hybrid phase was investigated with another method in more detail and also at lower temperatures. Thus, it was possible to observe superconductivity on that material in high magnetic fields up to 32 Tesla.

Phase diagram of the organic superconductor k-(BEDT-TTF)2Cu(NCS)2. At low temperatures the Fulde-Ferrell-Larkin-Ovchinnikov-Phase (FFLO) occurs. In the diagram it is visible as red interspace.